An economist is a man who states the obvious in terms of the incomprehensible. (Alfred A Knopf)
(This noun clause is a subject complement.)
(Not all agree this is a noun clause. See Note on the right.)
I could not understand this sentence whether it is noun clause or relative clause? Why?
The sequence "a man who states the obvious in terms of the incomprehensible" is NOT a noun clause or a relative clause; it's a NOUN PHRASE. It is SUBJECT COMPLEMENT to the subject "An economist".
Embedded in the noun phrase is the relative clause: "who states the obvious in terms of the incomprehensible" which is modifier of "man".